1. Field of the Disclosure
The disclosure generally relates to process control devices and, more specifically, to a system and method for controlling process control devices having multiple feedback control mechanisms.
2. Brief Description of Related Technology
A variety of control mechanisms may be used to control a valve/actuator assembly or other process control device. For instance, valve controllers typically use a form of feedback control to control the valve/actuator assembly. The feedback control relies on an error signal, which, in turn, is based on the difference between a desired set point signal and a feedback signal from a sensor, the feedback signal providing an indication of the operation of the valve/actuator assembly. The output of the valve controller is a correcting control signal, which in the past was a pneumatic signal proportional to the error signal.
While pneumatically driven actuators remain common, conventional control devices also incorporate non-pneumatic elements, such that the pneumatic signals are typically determined by electronic controllers, sensors or transducers (e.g., a digital valve controller). More particularly, the electronic controller executes a control routine that processes the error signal to generate a control signal, which may be converted to a current or other analog control signal, which, in turn, is fed to an electro-pneumatic converter, such as a current-to-pressure transducer within the valve controller to produce the pneumatic signal. Such control signal, whether analog or digital, may be referred to hereinafter in certain instances as a drive signal.
In most cases, a drive signal is generated from one of three types of control algorithms: (i) Proportional; (ii) a Proportional plus Derivative; and, (iii) a Proportional plus Derivative plus Integral. The Proportional (P) type control algorithm generates a drive signal directly proportional to the error signal. The Proportional plus Derivative (PD) type control algorithm generates a drive signal that is the sum of a signal proportional to the error signal and a signal that is proportional to the rate of change of the error signal. The Proportional plus Derivative plus Integral (PID) type control algorithm generates a drive signal that is the sum of a signal proportional to the error signal, a signal that is proportional to the rate of change of the error signal, and a signal that is proportional to the integral of the error signal.
The feedback signal that determines the error signal may be directly or indirectly indicative of the operation of the valve/actuator assembly. For example, one indication of the flow through the valve/actuator assembly involves a position sensor that generates a signal indicative of the position of the valve. To this end, valve/actuator assembly designs often include mechanical linkage between a position sensor and the valve to detect valve position. The position sensor is then coupled to the mechanical linkage to generate the valve position signal. A system having a feedback control mechanism based on a position sensor is often said to rely on position control.
Controllers for valves having a pneumatically driven actuator have utilized a pressure sensor as an alternative to position control. In this case, a pressure sensor provides an indication of flow through the valve because the actuator of the valve/actuator assembly has a spring, the compression of which is approximately proportional to the pressure applied thereto. This control mechanism is often referred to as pressure control.
Older process controllers, particularly those that predate microcomputers, often relied upon pressure control rather than position control. As these systems were modernized, certain components of the system were replaced or upgraded to include aspects of digital control. For example, a pneumatic pressure control for a valve could be replaced by a digital controller. To avoid a shutdown of the process, if not the entire plant, the new digital valve controller included pressure control to simplify the replacement and installation process. In this way, the older, pneumatic control could be replaced without having to replace or modify the valve/actuator assembly. As a result, the replacement of the old, pneumatic device, and the accompanying installation of the new, digital controller, avoided disturbing or discontinuing the operation of the valve or, more generally, the process. This replacement and installation process is known as hot cutover.
An example of a digital controller capable of hot cutover installation is the FIELDVUE™ Digital Valve Controller Type DVC5000 Series, specifically DVC5000f, Firmware Version 9, manufactured by Emerson Process Management-Fisher (Marshalltown, Iowa). The DVC5000f includes a pressure sensor and the associated pressure control routine in the interest of enabling hot cutover. The DVC5000f controller also includes a position sensor and the capability of selecting position control for those installations compatible with position control. As stated hereinabove, many installations involving a replacement did not support position control. In these cases, installation of the mechanical linkages between the valve and the position sensor would require either replacement or maintenance of the valve, most likely involving process shutdown. As a result, the DVC5000f would be first installed with pressure control as the feedback mechanism in operation. When a process or plant shutdown occurred, the valve/actuator assembly would be configured for position control. Thus, installation and use of a DVC5000f controller was often a two-step process, including an initial step of hot cutover to digital pressure control followed by a subsequent selection of position control once the linkages and other mechanical and/or valve components were installed during a shutdown. To enable the manual selection of position control, an interface made available to the operator provided an option to change a control parameter and thereby switch from pressure to position control.
During normal operation, control of a valve through position feedback is preferred over other feedback mechanisms that are more indirectly indicative of valve operation. Unfortunately, control using only position feedback is completely dependent upon a position sensing mechanism that is subject to failure. Past digital valve controllers have not provided for continued operation of the valve in the face of such failure events or other contingencies related to position sensor failure. As a result, and as a consequence of the nature of feedback control, a valve can be rendered inoperative by a faulty sensor despite the otherwise healthy condition of the valve. Once rendered inoperative, the valve, in turn, may cause an undesirable and unnecessary shutdown of the process or plant.